Spark plugs are an essential part of internal combustion engines and there are many prior art devices which propose variations in the configuration of either the ground electrode or center electrode.
By way of illustration, the U.S. Pat. No. 4,268,774 to Forkum, Jr., granted May 19, 1981 relates to a spark plug with a ground electrode having split or diverging prongs which continuously diverge into non-parallel angularly spaced apart end portions. The base or vertex region where the two prongs join together is so located relative to the plugs center electrode that a spark bridging the spark gap impinges first at the base region and then splits for simultaneous travel along the end portion. It is critical in the Forkum plug that the location of the base or vertex of the split be carefully located relative to the periphery of the end of the center electrode.
This requires sophisticated manufacturing equipment to split the ground electrode and preferably curve the prongs. This increases the cost of manufacture.
A further refined plug, as taught by Forkum in his U.S. Pat. No. 4,916,354 granted Apr. 10, 1990, has split prongs, with the end of at least one prong re-directed along the periphery of the center electrode. Again, the sophistication in manufacturing and assembling such plugs, in order to properly locate the prongs relative to the center electrode, is apparent.
Others have approached the re-design of spark plugs by concentrating on the configuration of the end of the center electrode. By way of example are the U.S. patents of Tanaka et al, U.S. Pat. No. 4,329,615 granted May 11, 1982; Yamada, U.S. Pat. No. 4,336,477 granted Jun. 22, 1982 and Strumbos, U.S. Pat. No. 4,910,428 granted Mar. 20, 1990.
The spark plug of Tanaka et al teaches a center electrode with a cut portion formed on one side of the center electrode end face opposing the bend in the ground electrode. As part of the prior art (FIGS. 4A, 4B and 5) Tanaka et al discloses that it is known to provide cross-shaped grooves in the end face of the center electrode which has the effect of decreasing the required voltage and also the contact area between the center electrode and the flame core ignited by the discharge. While the cooling effect is reduced, the location of the discharge at certain of the portions of the end face defined by the grooves does not provide the improved ignition performance otherwise desired.
The Yamada plug provides a grooved center electrode along the plane of the grounded electrode to promote the location of discharge gaps on the outer sides of the center electrode adjacent the aligned outer sides of the ground electrode.
The patent to Strumbos simply illustrates plugs with modified center electrodes including single or cross grooves as well as providing a thin coating to resist electrical and residue erosion.
Accordingly there is a need for a spark plug which avoids the criticality of Forkum's type of arrangement and yet provides for good ignition performance.